We cannot come in time. With every hoof-beat of the relentless ride Sauron's visions haunt him, taunt him; the black ships, the white city burning. Though I drive the men and the horses into the very ground, and bring the host of the Dead at my back… how can we ever come in time?
At last, in utter darkness, they climb the black hill to the black stone. Now I stake my claim; now must I hold them to their broken oath. And if I fail…
Dead eyes will see, though the night be as pitch. "Hal? Unfurl my standard."
And with that he bade Halbarad unfurl the great standard which he had brought; and behold! it was black, and if there was any device upon it, it was hidden in the darkness. Then there was silence...."
The Return of the King, LoTR Book 5, Ch 2, The Passing of the Grey Company.